ABC News said sources had told it that intelligence officials “now have a clearer picture of just how the attack was conducted”.

Agencies reportedly now believe the toxin – identified as a fourth generation nerve agent called Novichok – was used in a “dust-like powdered form” and that it circulated through the vents of Colonel Skripal’s BMW.

While the method of assumed delivery keeps changing, one thing remains clear: Novichok is relatively easy to deliver and is extremely debilitating, even deadly. There are also new reports coming out Sunday that, in addition to its ease of use, the toxin is also being stockpiled by Russia.

UK Prime Minister Theresa May told MPs that Porton Down – Britain’s military research base – identified the substance used as being part of a group of military-grade nerve agents known as Novichok developed by the Soviet Union.

Mr Johnson told the BBC’s Andrew Marr Show: “We actually have evidence within the last 10 years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination, but has also been creating and stockpiling Novichok.”

He said Russia’s reaction to the incident “was not the response of a country that really believes itself to be innocent”.

Skripal and his daughter were found slumped on a bench on March 4 while on a shopping trip to Salisbury, a town roughly 90 miles outside London. They are both critically ill and still recovering from the attack. A police officer who attended to them also fell ill and is recovering. British police are investigating the attack as an attempted murder.